Manufacture of optical lens elements has long involved complex, slow, expensive and tedious procedures. Although concerted efforts to improve existing methods and apparatus of manufacture have been undertaken, prior art methods still have many disadvantages and problems.
Precision optical elements require polished surfaces of exacting figure and surface quality. The surfaces demand fabrication in proper geometric relation to each other and, where the elements are used in transmission applications, they will be prepared from a material of controlled, uniform, and isotropic refractive index. For some applications non-isotropic refractive index materials have been known.
Precision optical elements of glass are customarily produced by means of one of two complex, multi-step processes. In one process, a glass batch is melted in a conventional manner and the melt formed into a glass body having a controlled and homogeneous refractive index. Thereafter, the body may be reformed utilizing well-known repressing techniques to yield a shape approximating that of the desired final article. The surface figure and finish of such an intermediate product are not suitable for image forming optics. The intermediate article is fine annealed to develop the proper refractive index, and the surface figure thereof is improved by means of conventional grinding practices. Another method involves forming a glass melt into a bulk body which is promptly fine annealed and subsequently cut and ground to articles of a desired configuration.
Both of the preceding processes have similar limitations. The surface profiles that are produced through grinding are normally restricted to conic sections, such as flats, spheres, and parabolas. It should be understood that other shapes, in particular, general aspheric surfaces are difficult to grind. In both processes, the ground optical surfaces are polished employing conventional, but complicated polishing techniques which are intended to improve surface finish without compromising the surface figure. In the case of the aspheric surfaces, such polishing requires highly skilled and very expensive hand-working. A final finishing operation, such as, for example, edging is also commonly required. Edging ensures that the optical and mechanical axes of a spherical lens coincide. Edging, however, does not improve the relationship of misaligned aspheric surfaces, if such are present, which factor accounts, at least in part, for the difficulty experienced in grinding such lenses with precision.
Direct molding of lenses to a finished state could, at least in principle, eliminate the grinding, polishing and edging operations, which are especially difficult and time consuming for aspheric lenses. Such molding processes are employed for fabricating plastic lenses. However, existing plastics suitable for optical applications are available in a limited refractive index and dispersion range. In addition, many plastics scratch easily, display birefringence and are prone to the development of yellowing and haze. Abrasion resistant and anti-reflective coatings have been used but have failed to fully solve these problems with plastics. Further, plastic optical elements are subject to distortion from mechanical forces, humidity, and heat. Both the volume and refractive index of plastics vary substantially with variations in temperature thereby limiting the temperature interval over which plastics are useful.
The properties of glass render it generally superior to plastic for optical applications. Conventional hot pressing of glass, however, does not provide the exacting surface figures and surface qualities demanded for image forming optics. The presence of chill wrinkles in the surface and surface figure deviations are chronic problems. Similar problems can be encountered in conventional repressing techniques as noted above.
Numerous means and devices have been employed to correct the shortcomings of conventional hot glass pressing processes and apparatuses. Among these are special pressing apparatuses utilizing isothermal pressing, i.e., pressing using heated molds and preheated glass so that the temperatures under which the pressing step is carried out vary only slightly across the glass preform during the pressing interval, and using a gaseous environment inert to the glass and mold materials during the pressing operation. In addition, special materials to construct the molds, special glass compositions and molding process parameters have been developed and used in an effort to improve the quality of lenses as well as other optical elements which are directly pressed.
Various patents related to mold and glass manufacture are noted and described below, and all of these patents are hereby expressly incorporated herein by reference. U.S. Pat. No. 2,410,616 describes an early apparatus and method for molding glass lenses. The molds are capable of being heated and the temperatures thereof controlled within narrow ranges compatible which the glasses being molded. An inert or reducing gas environment, preferably hydrogen, is used in contact with the mold surfaces to inhibit oxidation thereof. A flame curtain, normally burning hydrogen, over the opening of a chamber enclosing the molds to prevent the entrance of air thereinto is described.
U.S. Pat. No. 3,833,347 is similarly directed to an apparatus and method for press molding glass lenses. The molds can be heated and the temperature controlled. An inert gas surrounds the molds to preclude oxidation. This patent discloses the use of mold surfaces composed of glass-like carbon which are distinguished from metal dies that were stated to produce lens surfaces not suitable for photographic applications. The method described comprises eight steps including placing a chunk of glass into a mold, evacuating the chamber surrounding the mold and introducing a gas therein, raising the mold temperature to about the softening point of the glass, applying a load to the mold to shape the glass, reducing the temperature of the mold to below the transformation temperature of the glass while maintaining the load on the mold to prevent distortion of the shaped glass body, removing the load, cooling the mold to about 300.degree. C. to inhibit oxidation of the glass-like carbon, and lastly opening the mold. This patent asserted that lenses so produced were essentially strain-free without the need for further annealing.
A similar teaching of an apparatus and method for transfer molding glass lenses employing glass-like carbon surfaces on the mold is found in U.S. Pat. No. 3,844,755. The use of mold coatings to enhance the surface quality of the pressings, to improve mold durability, and act as a parting agent from the molten glass is suggested in U.S. Pat. No. 3,244,497. This patent describes a lens blank molding apparatus wherein a temperature controlled plunger and an insulated mold base offering controllable heat transfer to a supporting press table are described. The apparatus, however, is designed for pressing relatively thin lens blanks, which factor is an important contributor to the temperature control attainable with the apparatus.
U.S. Pat. No. 4,481,023 describes an alternative molding apparatus for direct pressing of lenses of optical quality. Temperature control of the molding surfaces is also provided, and the apparatus is designed for pressing at relatively high glass viscosities of 10.sup.8 -10.sup.12 poises. This corresponds to a relatively low pressing temperature, which helps to reduce difficulties stemming from non-uniform heat flow.
U.S Pat. No. 3,244,497, supra, teaches refractory coating selected from the group consisting of refractory nitrides, borides, carbides, and oxides. Coatings no thicker than approximately half of the wavelength of visible light, e.g. 0.5 microns, are suggested in order that the coating faithfully reproduce the mirror finish of the underlying mold surface.
U.S. Pat. No. 3,900,328 generally describes molding glass lenses utilizing molds fabricated from glass-like carbon. This reference discloses placing a portion of heat softened glass into a cavity of a mold prepared from glass-like carbon, applying appropriate amounts of heat and pressure to the mold while maintaining a non-oxidizing atmosphere in the vicinity of the mold, cooling and opening the mold, and then removing the finished lens from the mold.
U.S. Pat. No. 4,168,961 describes a method for precision molding of optical glass elements wherein a mold having mold surfaces of a silicone carbide/glassy carbon mixture is taught. The patent suggests that elements molded employing such mold material exhibit high surface quality and surface accuracy. However, molding while maintaining a controlled atmosphere is required to avoid oxidation of this material, a condition which substantially reduces the practical economical value of the method.
Press forming optical lenses from hydrated glass is taught in U.S. Pat. No. 4,073,654. The process involves placing granules of hydrated glass into a mold, drawing a vacuum on the mold, heating the mold to a sufficiently high temperature to sinter the granules while the mold is sealed to prevent escape of water vapor therefrom, applying a load to the mold, releasing the load from the mold, and opening the mold. Glass-like carbon, tungsten carbide, and alloys of tungsten are suggested mold materials.
European patent application No. 19342 discloses isothermal pressing of glass lenses at temperatures above the softening points of the glasses employed, i.e. at temperatures where the glasses exhibit viscosities of less than 10.sup.7.6 poises.
U.S. Pat. No. 4,139,677 teaches precision molding of optical glass elements in a mold having molding surfaces formed of silicon carbide or silicon nitride. This method allegedly provides good surface quality and configuration, however, it requires maintaining an oxygen-free atmosphere within the molding chamber to avoid oxidation of the mold coatings.
U S. Pat. No. 4,747,864 describes glass optical elements formed by a direct molding process at glass viscosities in the range of 10.sup.8 -10.sup.12 poises. Selected moldable alkali aluminofluorophosphate optical glasses are pressed to an optical surface finish in air utilizing an optically smooth titanium nitride molding surface, the surface being provided, for example, as a surface coating on a stainless steel mold or on a nickel chromium alloy mold supporting an electroless nickel base coating.
U.S. Pat. No. 4,734,118 describes a mold for pressing a glass preform which has an overall geometry similar to the desired final lens. The top and bottom mold dies having mold cavities which match the configuration of the final lens. A glass preform is heated to the molding temperature while the mold parts are separately heated. The mold parts are brought together against a ring having a thickness which governs the thickness of the lens to be molded. The volume of glass that is put into the molding cavity is controlled by measuring its mass. The reference teaches a variety of mold surface materials such as 400 series stainless steels, electroless nickel, beryllium nickel alloys, tungsten carbide, alloys of noble metals such as platinum, rhodium and gold and fused silica. The patent teaches that the mold material itself is not critical but must be capable of accepting a good surface finish.
In order to realize the economic advantages of employing direct molding techniques for products such as aspheric lenses, factors relating to the service life of the molds employed for the pressing operation are most significant and must be taken into strict account. The machining of aspheric shapes in molds renders the molds relatively expensive, particularly since very hard and durable mold materials are generally required. This is especially true for molding processes involving low-temperature, high-viscosity molding, since higher molding stresses are involved.
Primary factors affecting mold life include chemical reactions occurring between the hot mold and the glass to be molded, and between the hot mold and the atmosphere. The latter factor is particularly significant when rapid production rates that prohibit cooling of the shaped lens in the mold are desired. Prior art approaches have suggested using a controlled atmosphere for molding to avoid oxidation or other degradation of the mold surfaces, however, such steps are inconsistent with rapid and economical optical element production.
The development of direct glass element molding has been substantially assisted by the discovery of new glass compositions which can be molded at relatively low pressing temperatures while not being subject to attack by moisture in the manner usual for soft glasses. U.S. Pat. No. 4,362,819 discloses examples of alkali aluminofluorophosphate glasses useful for such applications. However, pressing of such glasses at economical rates has been difficult because of limited compatibility between these glasses and conventional mold materials.